These studies will investigate, in the rat, the cardiac response to aging in the unstressed state and with stress, pressure overloading or exercise training. The studies will correlate changes in ventricular function and rates of energy metabolism in hearts isolated and perfused in vitro. Ventricular function will be determined in the isolated heart at several levels of left atrial filling pressure. Rates of energy metabolism will be determined at a low level of cardiac work and at a high level of cardiac work. These studies will include measuring the rate of glycolysis, glucose oxidation, lactate production and rates of fatty acid uptake and oxidation, as well as oxygen consumption. Control of these pathways, as well as the levels of creatine phosphate and adenine nucleotides. In addition, the effect of age and stress on the response of the heart to ischemia will be determined. Ischemia will be induced by decreasing the rate of coronary flow to the whole heart in the isolated, working heart preparation.